1. Field of the Invention
The invention relates to a surface inspection apparatus for determining whether defects, such as minute protrusions or recesses, are present on a flat, smooth surface, such as a surface of an automobile body coated with a paint film.
2. Description of the Related Art
On production lines of automobile factories, for example, an inspecting operation to check if defects, such as minute protrusions or recesses, are present on a paint film surface of an automobile body is conducted through direct, visual inspection of the paint film surface by a worker.
Also, four methods, namely, 1) inspection using diffusion-type flat lighting, 2) inspection using parallel-beam lighting, 3) inspection using diffusion-type flat lighting with a pattern, and 4) inspection using a plurality of small illuminators arranged in a row, have been implemented as methods for inspecting a surface state or condition.
A surface state evaluation apparatus that quantitatively evaluates the painting or coating quality is disclosed in Japanese Patent Application Publication No. 2003-28805 (JP-A-2003-28805). The surface state evaluation apparatus disclosed in JP-A-2003-28805 includes illuminating means having a plurality of illuminators that are positioned such that light beams emitted from the respective illuminators are incident upon an object to be measured at different incidence angles, control means for switching the illuminators of the illuminating means in time sequence so that the object is irradiated with the illumination light emitted in turn from the illuminators, imaging means for capturing reflected light beams from the object so as to create corresponding images, and evaluation value calculating means for calculating evaluation values representing reflection characteristics of a surface of the object based on changes in the incidence angle of the illumination light.
Also, an inspection apparatus that conducts an inspection on an object for defects, using image data captured by a camera, is disclosed in Japanese Patent Application Publication No. 11-237210 (JP-A-11-237210). The inspection apparatus disclosed in JP-A-11-237210 includes a dispersion optical system that disperses incident light in different directions in correspondence with three different wavelength ranges, an imaging device that images light of each of the wavelength ranges into which the incident light is dispersed, illuminating means having three illuminators to which the three different wavelength ranges into which the incident light is dispersed by the dispersion optical system are assigned, and inspecting means for turning on these illuminators at the same time so as to enable the imaging device to image the light of each wavelength range, and conducting an inspection on the object for detects, based on image data obtained by the imaging device.
The above-mentioned visual inspection conducted by a worker imposes a great physical burden on the worker, and it is thus difficult for the worker to continue the inspecting operation for a long time, which makes it difficult to increase the productivity. Also, the visual inspection depends largely on the ability or efficiency of the worker, and the inspection quality varies to a great extent from one worker to another, which makes it difficult to maintain uniform quality.
In the case of the inspection using the diffusion-type flat lighting as mentioned above at 1), if the area of the surface illuminated is increased, the light diffracts in all directions at minute protrusions or recesses, such as blisters on a painted surface, and the protrusions or recesses do not appear as feature points, resulting in reduced inspection capability. If the area of the surface illuminated is reduced so as to avoid diffraction of light, an area or range inspected by one inspecting operation is considerably narrowed, and the inspection of a coated surface having a large area (for example, about 10 square meters), such as that of an automobile body, cannot be completed within a tact time (for example, about 60 seconds) of a conveyor carrying the object to be inspected.
If the inspection using the parallel-beam lighting as mentioned above at 2) is employed, the directivity of the light can be controlled, and therefore the diffraction of the light, which is a shortcoming of the above type of lighting 1), can be avoided. However, if the object to be inspected has a curved surface, the area over which specularly reflected light is received by a light-receiving portion is reduced, and the field of view covered by one inspecting operation is considerably narrowed. Thus, as in the case 1) above, the inspection of a surface having a large area, such as that of an automobile body, cannot be completed within a tact time of the conveyor. Also, metallic powder or mica contained in a coating may be erroneously detected as defects, depending on the orientation of the metallic powder or mica.
In the case of the inspection using the diffusion-type flat lighting with a pattern as mentioned above at 3), the detection capability is improved as compared with the above type of inspection 1), and the inspection speed is increased as compared with the above type of inspection 2). However, there is a problem in the detection accuracy at boundaries between light and dark portions of the pattern. To solve this problem, two or more cameras are mounted in one unit, so as to shift the phase of the light and dark pattern. However, the same number of image processing boards, personal computers, lenses, and other devices as that of the cameras is required, resulting in increase of the investment cost.
In the case of the inspection using a plurality of small illuminators arranged in a row as mentioned above at 4), the plurality of illuminators need to be turned on one by one so as to enable a camera to take pictures, with respect to one point on the object to be inspected. Accordingly, the movement of the object to be inspected needs to be stopped each time the inspection is conducted on one point, resulting in reduction of the inspection speed. Also, if high-speed photographing is conducted, the quantity of light is reduced, and therefore the SN ratio is reduced.
In the case of the technology described in JP-A-2003-28805, it is necessary to switch the plurality of illuminators in time sequence with respect to one point on the object to be inspected, and cause the imaging means to image the reflected light each time the illuminators are switched from one to another. Thus, it takes much time to capture images, resulting in reduction of the inspection speed.
In the case of the inspection apparatus described in JP-A-11-237210, reflected light from a defect containing surface of an object to be inspected may not be imaged by the imaging means, depending on how each of the illuminators is positioned, resulting in reduced inspection accuracy.
Also, if the relationship in angular position between the object to be inspected and the illuminating means, or the relationship in angular position between the object to be inspected and the imaging device is changed, reflected light that is specularly reflected by the object may not be imaged by the imaging means. Thus, the inspection apparatus is susceptible to inclinations or changes in the angular positions of its components, and the illuminating means and the imaging means are required to be positioned with high accuracy with respect to the object to be inspected.
Accordingly, it is difficult to inspect the entire surface of an object, such as an automobile body, having a considerably large area, while keeping the illuminating means and the imaging device in their correct angular positions, and the inspection apparatus cannot be appropriately used for detection of defects in the form of minute protrusion or recesses.
Referring to FIG. 25 through FIG. 28C, the relationship between the ability to detect a defect Wb and the inspection speed in the related art of the invention will be explained.
In FIG. 25-FIG. 28C showing the manners of detecting a defect Wb, an illuminating means is positioned so as to be opposed to an inspection surface Wa of a body W to be inspected, and the inspection surface Wa is irradiated by the illuminating means, so that reflected light that is reflected by the inspection surface Wa is captured and imaged by imaging means, for detection of a defect Wb.
For example, when a combination of an area camera 201 and an illuminating means 202 having a relatively large illumination size is used, as shown in FIG. 25, a relatively large area or range of the inspection surface Wa can be photographed at a time by the area camera 201, and the range photographed per unit time can be increased.
However, if the illumination size of the illuminating means is relatively large with respect to the defect Wb, as is the case with the illuminating means 202, the area camera 201 receives reflected light C1 as a result of specular reflection of illumination light by portions of the inspection surface Wa other than the defect Wb, and also receives reflected light C2 as a result of specular reflection of illumination light by inclined portions of the defect Wb. Accordingly, the defect Wb may be embedded in the reflected light beams C1, C2, and the defect Wb may not clearly appear in an image captured by the area camera 201.
If, on the other hand, an illuminating means 203 having a relatively small illumination size with respect to the defect Wb is used, as shown in FIG. 26, for example, reflected light C3 as a result of specular reflection of illumination light by inclined portions of the defect Wb is prevented from being captured and imaged by the area camera 201.
Accordingly, the quantity of the reflected light that is reflected by the defect Wb and imaged by the area camera 201 can be made smaller than the quantity of the reflected light that is reflected by other portions of the inspection surface Wa than the defect Wb and imaged by the area camera 201. In the resulting image, the defect Wb appears to be dark as compared with the other portions of the inspection surface Wa than the defect Wb. Thus, a contrast can be produced between the defect Wb and the remaining portions of the inspection surface Wa, and the defect Wb can be indicated clearly.
However, the illuminating means 203 has a narrower illumination range than the illuminating means 202, and the range that can be photographed per unit time is small. Therefore, it takes too much time and impractical to detect defects Wb in the form of minute protrusions or recesses on the inspection surface Wa of the body W, such as an automobile body, having a considerably large area.
In view of the above situation, a line camera 211 may be used in place of the area camera 201, and combined with the illuminating means 203, as shown in FIG. 27A-FIG. 27C, for example. In operation, the line camera 211 and the illuminating means 203 are moved relative to a body W to be inspected, so as to scan an inspection surface Wa of the body W. In this case, the range that can be photographed per unit time can be increased as compared with the case where the area camera 201 is used, because the line camera 211 generally has a greater frame rate than the area camera 201, and is able to capture image data in a shorter time. Accordingly, the defect Wb can be detected in a relatively short time.
However, the line camera 21 captures an image over an extremely small range as measured in the direction of movement of the camera 211. Therefore, if the angles of the line camera 211 and the illuminating means 203 relative to the inspection surface Wa deviate from preset reference angles, as shown in FIG. 28 by way of example, specularly reflected light C4 cannot be captured and imaged by the line camera 211.
Thus, the acceptable degrees of inclination of the line camera 211 and the illuminating means 203 relative to the inspection surface Wa of the body W to be inspected are small. Namely, the inspection apparatus of the related art is susceptible to changes in the angles of the line camera 211 and the illuminating means 203 relative to the inspection surface Wa of the body W, and has a narrow adaptive range with respect to changes in these angles.